Paper webs or sheets, sometimes called tissue or paper tissue webs or sheets, find extensive use in modem society. These include such staple items as paper towels, facial tissues and sanitary (or toilet) tissues. These paper products can have various desirable properties, including wet and dry strength, softness, and lint resistance.
Strength is the ability of the product, and its constituent webs, to maintain physical integrity and to resist tearing, bursting, and shredding under use conditions, particularly when wet.
Softness is the tactile sensation perceived by the consumer as he/she holds a particular product, rubs it across his her skin, or crumples it within his her hand. This tactile sensation is provided by a combination of several physical properties. Important physical properties related to softness are generally considered by those skilled in the art to be the stiffness, the surface smoothness and lubricity of the paper web from which the product is made. Stiffness, in turn, is usually considered to be directly dependent on the dry strength of the web and the stiffness of the fibers which make up the web. In particular, as dry strength increases, softness decreases.
Lint resistance is the ability of the fibrous product, and its constituent webs, to bind together under use conditions, including when wet. In other words, the higher the lint resistance is, the lower the propensity of the web to lint will be.
The dry strength of paper products should be sufficient to enable manufacture of the product and use of the product in a relatively dry condition. Increases in dry strength can be achieved either by mechanical processes to insure adequate formation of hydrogen bonding between the hydroxyl groups of adjacent papermaking fibers, or by the inclusion of certain dry strength additives. Such dry strength additives are typically natural or synthetic polymers. Exemplary dry strength additives include: starch and starch derivatives, polyvinyl alcohol, and polyacrylamide.
Wet strength is a desirable attribute of many disposable paper products that come into contact with aqueous fluids in use, such as napkins, paper towels, household tissues, disposable hospital wear, etc. In particular, it is often desirable that such paper products have sufficient wet strength to enable their use in a moistened or wet condition. For example, a moistened tissue or towel may be used for body or other cleaning. Unfortunately, an untreated cellulose fiber assemblage will typically lose 95% to 97% of its strength when saturated with water such that it cannot usually be used in the moistened or wet condition.
Historically, one approach to providing wet strength to paper products is to incorporate additives in the paper product which contribute toward the formation of interfiber bonds which are not broken or, for temporary wet strength, which resist being broken, by water. A water soluble wet strength resin may be added to the pulp, generally before the paper product is formed (wet-end addition). The resin generally contains cationic functionalities so that it can be easily retained by the cellulose fibers, which are naturally anionic.
A number of resins have been used or disclosed as being particularly useful for providing wet strength to paper products. Certain of these wet strength additives have resulted in paper products with permanent wet strength, i.e., paper which when placed in an aqueous medium retains a substantial portion of its initial wet strength over time. Exemplary resins of this type include urea-formaldehyde resins, melamine-formaldehyde resins and polyamide-epichlorohydrin resins. Such resins have limited wet strength decay.
Permanent wet strength in paper products is often an unnecessary and undesirable property. Paper products such as toilet tissues, etc., are generally disposed of after brief periods of use into sewage systems and the like. Clogging of these systems can result if the paper product permanently retains its wet strength properties. Therefore, manufacturers have more recently added temporary wet strength additives to paper products for which wet strength is sufficient for the intended use, but which then decays upon soaking in water. Decay of the wet strength facilitates flow of the paper product through septic systems. Numerous approaches for providing paper products claimed as having good initial wet strength which decays significantly over time have been suggested.
One type of temporary wet strength additive are aldehyde containing resins exemplified by COBOND 1000, an aldehyde functionalized cationic starch commercially available from the National Starch & Chemical Corp. of Bloomfield, N.J., and PAREZ 631 NC and PAREZ 750A, aldehyde functionalized cationic polyacrylamides commercially available from Cytec Industries, Inc. of West Paterson, N.J.
Exemplary patents describing paper products having temporary wet strength include: U.S. Pat. No. 4,981,557, issued to Bjorkquist on Jan. 1, 1991; U.S. Pat. No. 5,690,790, issued to Hedlam, et al. on Nov. 25, 1997; and U.S. Pat. No. 5,723,022, issued to Dauplaise, et al. on Mar. 3, 1998. While all of these patents describe paper products having a decay in strength with time after exposure to water or an aqueous solution, none of them describes low density paper products having a combination of short term maintenance of strength after exposure to water, decay in strength with time after exposure to water and softness as would be particularly desirable for paper products that are used for toweling, sanitary tissue, and the like. In particular, the paper products described by the above-identified patents have dry tensile properties that would suggest a need for improved softness or, in the absence of any disclosure of dry tensile properties, a need for improved short term maintenance of dry strength properties on exposure to water.
Thus, there is a continuing need for improvements in paper products that are used for toweling, sanitary tissue, and the like. In particular, there is a need for paper products that maintain a greater percentage of their dry strength when they are first wetted, while, on further exposure to water or an aqueous solution, showing a substantial decay from their initial wet strength. There is a further need for paper products having such desirable wet strength properties that are also soft and lint resistant.